During period 1 (June 6 to 9), emissions were recorded during 4 consecutive 24-h periods, whereas during each of periods 2 to 4 (June 26 to 30, July 31 to August 4, and September 4 to 8, respectively) emissions were recorded during 5 consecutive 24-h periods. The permeation tubes used within this experiment contained approximately 2.4 g of SF6 gas at filling (March 2011) and were incubated at 39°C until they were orally administered to cows on May 27 (10 d before the first experimental period). Prior to this, the release rate of SF6 from each permeation tube was determined via weekly gravimetric weighing over an 8-wk period to produce an 8-point regression curve (R2 >0.999). The mean release rate of SF6 from the permeation tubes at the start of the experiment was 5.40 (±0.82 SD) mg/d. To correct for the known decline in the rate of release of SF6 from the permeation tubes within the rumen during the course of the experiment, 10 surveillance tubes, with similar release rates as the tubes used in the experiment, were maintained at 39°C in an incubator and monitored weekly until 6 wk after the completion of the final measurement period. The release rate of SF6 from the surveillance tubes was found to decline by 0.15% per day, and this value was subsequently used to adjust the release rates for the experimental tubes during periods 2, 3, and 4, as described by Lassey et al. (2001).